This invention relates to a hard magnetic material, and more particularly to a rare earth cobalt magnet.
Some copper containing rare-earth cobalt materials are known to exhibit high coercive force independent of their grain size. This phenomenon is believed to originate from domain wall stabilization due to fine copper-rich nonmagnetic precipitates. The term "bulk hardening" will be used throughout the specification to denote such an effect. Thus "bulk hardening" means "to invest rare earth cobalt alloys with high coercive force by adding copper". No additives other than copper have been found to cause the effect to the same extent as copper.
One of advantages of the bulk hardening method in producing rare earth cobalt magnets is that one need not pay any special attention to grain size control problem which is often essential in the other methods. Thus, bulk hardening affords easy production.
Shortcomings of the bulk hardening method include severe reduction of saturation induction, which is inevitably caused by a rather heavy incorporation of the nonmagnetic element. The fact that the degree of bulk hardening depends on the amount of copper has been noted for years.
However, the other factors influencing bulk hardening have been noted to a lesser degree. It is worth mentioning here that the degree of bulk hardening greatly depends on kind of rare-earth or rare-earth combinations employed and on rare-earth to cobalt (plus copper) ratio.
Cerium cobalt and samarium cobalt (iron may be added) with 1:5 stoichiometry are good examples in which the bulk hardening has been successfully employed to obtain excellent magnets with maximum energy product of 12 MG.Oe and residual induction of 7000 G. In contrast, PrCo.sub.5 exhibits no significant bulk hardening.
U.S. Pat. No. 3,560,200 claims that bulk hardening effectively works in nonstoichiometric compositions in which rare-earth to cobalt (plus copper) ratio falls between 1:5 to 1:8.5 "to a comparative degree" with respect to the 1:5 stoichiometry cases. It is generally expected that increasing the relative amount of cobalt to rare earth increases intrinsic saturation induction, and thus improves maximum energy product. However, it has been generally believed that the increase in the relative amount of cobalt to rare earth weakens the bulk hardening effect, thus requiring more copper addition which in turn diminishes intrinsic saturation induction. Thus, the extension of the composition to the Co-rich side has been considered to bring a similar characteristics, at most to 1:5 stoichiometric cases.
Strnat, in a review article in IEEE Trans. on magnetics vol. MAG-8, No. 3, pp514 (1972), states that the attained maximum energy product of 12 MGOe (for 1:5 Ce-Co and Sm-Co cases) probably represents maximum obtainable with the bulk hardening method. However, since bulk hardening is greatly affected by the kind of rare earth employed, there is no reason to deny that special combinations of rare earth elements would possibly enhance bulk hardening even for the nonstoichiometric compositions.
An object of the present invention is to provide a novel and improved magnetic materials having high saturation induction, high coercive force and high maximum energy product.
Another object of the invention is to provide an improved magnetic materials having the CaCu.sub.5 type hexagonal crystal structure and being characterized by the improved characteristics.
Further object of the invention is to provide a novel rare earth cobalt magnet made by sintering.
These objects are realized by providing the magnetic materials according to the invention having the compositions of Sm.sub.u Ce.sub.1.sub.-u (Co.sub.1.sub.-x.sub.-y Fe.sub.x Cu.sub.y).sub.z in which 0.3.ltoreq.u.ltoreq.1.0, 0.ltoreq.x.ltoreq.0.1, 0.09.ltoreq.y.ltoreq.0.18 and 6.0.ltoreq.z.ltoreq.7.5.